This invention relates generally to electrical interconnections between electronic modules and, more particularly, to stacked circuit substrates or modules, such as multi-chip modules (MCMs), having closely spaced interconnections that can be readily removed for repair, replacement and reconfiguration of individual modules.
The invention is applicable in particular to memory modules and electronic back planes, where the interconnect structure is not very complex, and the substrates or modules have a high degree of common signals or electrical potentials. The invention is also applicable to demountable and repairable assemblies of supercomputer modules.
Disclosed in Eichelberger et al U.S. Pat. No. 4,783,695 and related patents, a high density interconnect (HDI) structure offers many advantages in the compact assembly of electronic systems which include a number of integrated circuit "chips." HDI fabrication techniques can be employed to form multi-chip modules (MCMs). Very briefly, a substrate is provided. The substrate is typically made of ceramic. However, plastics or composites of various types may also be employed. Individual cavities or one large cavity having appropriate depths at the intended locations of the various chips are prepared. The various "chips" and other components are placed in their desired locations within the cavities, and adhered by means of a thermoplastic adhesive layer. A multiple layer interconnect overcoat structure is then built up to electrically interconnect the components into a functioning system.
The process of forming the multiple layer interconnect overcoat structure begins with laminating a polyimide dielectric film, which may be DuPont Kapton, polyimide, about 12.5 to 75 microns thick, across the top of the chips, other components and the substrate. The as-placed locations of the various components and contact pads thereon are then determined, and via holes are laser drilled in the Kapton film in alignment with the contact pads. A metallization layer is deposited over the Kapton film layer and extends into the via holes to make electrical contacts to the contact pads disposed thereunder. This metallization layer may be patterned to form individual conductors during the process of depositing it, or may be deposited as a continuous layer and then patterned using photoresist and etching. Additional dielectric and metallization layers are formed as required to make all of the desired electrical connection among the chips supported by the substrates.
For even higher density, individual HDI substrates or modules have been stacked to form what may be termed a three-dimensional high density interconnect structure. Examples are disclosed in Eichelberger et al U.S. Pat. Nos. 5,019,946 and 5,107,586. The individual HDI modules are vertically interconnected by providing electrical contact pads on the side surfaces of the individual HDI substrates, so that, when the individual substrates are stacked, the stacked configuration has a side surface presenting a number of contact pads which can then be interconnected to form interconnections between the substrates.
To make interconnections on the side surface, a fabrication process comparable to the above-summarized lamination process for building the multi-layer interconnection overcoat structure of the individual modules is employed, including laser ablating vias down to the edge pads, metallizing and patterning the metal.
This process is both time consuming and costly. The resultant assembly is essentially a bonded stack that is not easily repairable or demountable for replacement of defective modules.
As an alternative, wire bonding of the edge pads of such stacked modules has been performed. However, wire bonding increases interconnect length and the wire bonds are difficult to remove without damage to the modules. In addition, the process requires specialized wire bonding equipment which can operate on the edges of the substrates.
To avoid wire bonding to the substrate edges, pyramidal structures may be formed with a succession of module sizes, so that one tier of interconnects can be wire bonded from the upper major surface of one module to interconnect pads on the upper major surfaces of the adjacent tier or tiers. Such a pyramidal structure imposes a penalty in that the area of the upper modules in the stack must be reduced.
Another solution that has been proposed is the use of anisotropic conductive adhesive films. As is described for example in a literature reference Gilleo, K. (Alpha Metals, Jersey City, N.J.), "Assembly with Conductive Adhesives," Proceedings of Surface Mount International 1994, San Jose, Calif., pp. 279-288, anisotropic conductive adhesives provide unidirectional conductivity in the direction of the vertical, or Z-axis. This uni-directional conductivity is achieved by using a relatively low volume loading of conductive filler in a dielectric polymer bonding agent. The low volume loading is insufficient for inter-particle contact and prevents conductivity in the X-Y plane of the adhesive. The material, in film or paste form, is interposed between the surfaces to be connected. Application of heat and pressure causes conductive particles to be trapped between opposing conductor surfaces. Once electrical continuity is produced, the dielectric polymer binder is hardened by thermally initiated chemical reaction (thermosets) or by cooling (thermoplastics). The hardened polymer holds the assembly together and helps maintain the pressure contact between conductors and particles.
For assembling a three-dimensional stack of modules, anisotropic conductive adhesive films are placed between adjacent modules to establish electrical connections between pads on the bottom of one module and on the top of the next lower module in a stack. For the bottom connection, within or on the individual module substrate, interconnects are extended bottom pads through via holes or wrap around interconnects. As in the case of the three-dimensional high density interconnect structure summarized above, the resultant assembly is essentially a bonded stack that is not easily repairable or demountable for replacement of defective modules.
Soldered interconnects such as ball grid arrays (BGA) offer similar possibilities. Ball grid arrays also have the disadvantages of relative difficulty in assembly and repair, as well as a lack of demountability. Further, it is difficult to achieve a high density of interconnect even with micro-ball grid arrays. A 50 mil pitch is typical, with perhaps a 10 mil pitch being the lower limit which can be achieved with a ball grid array.
Also relevant in the context of the present invention is the prior art use of conductive elastomeric materials for providing electrical interconnection. Conductive elastomeric materials typically employ conductive particles, such as silver, carbon, nickel and gold, dispersed throughout a rubber-like material, such as silicone rubber material or certain polyurethanes.
Like anisotropic conductive adhesives, conductive elastomeric materials are typically anisotropic, meaning they are conductive, when compressed, only in the direction of compression, compression causing the conductive particles to contact each other. Generally, raised contact pad areas are provided, designed to give a 20% compression, or better. Sheets of anisotropic conductive elastomeric materials accordingly can be employed to achieve selective connection zones, while maintaining virtual open circuits to adjacent lines or pads.
Such conductive elastomeric materials (as well as anisotropic conductive adhesives) have been used for years to interconnect liquid crystal display cells and printed circuit boards to other electronics. In the case of connections to liquid crystal display cells, high resistance connections (e.g. hundreds of Ohms) are acceptable, since such constitutes a small additional series resistance. Carbon fillers in silicone rubber have been used for these applications for years. In other applications where a lower resistance interconnect is required, silver or gold fillers are used in the elastomeric interconnect materials.
A disadvantage of such use of sheets of anisotropic conductive elastomeric material deserving mention in the context of the present invention is that they can take a compressive "set," and often are not reusable following a repair operation.
To eliminate multiple particle connection regions that are intrinsic to the use of elastomeric materials filled with conductive particles, a structure known as metal ring banding can be employed. In metal ring banding, a plurality of conductive rings are provided around an elongated silicone core. The metal rings do not touch, and so maintain electrical isolation. When compressed, selected areas are connected. Metal ring banding around a silicone core is limited in the thickness of the core material, which is generally in the 15-20 mil region and up.
Another interconnection technique which deserves brief mention in the context of the subject invention is the use of the Zebra strip connectors, where wires are arrayed in a vertical orientation between PC boards and the like. These wires are on a 50 mil or greater pitch. The connections are made by pressure and are not widely used, due to the large containment structures employed to keep from bulging. Zebra strip connectors are generally used to make connections from one MCM to another, but without making connections to the edges of the MCMs. Zebra strip connections are not well suited to stacking modules, unless a backplane is provided.